Synthetic plastics materials have long been used for the packaging of foods and other materials which need protection from handling and from moisture. However, in recent years, it has become appreciated that, in addition, many foods and other sensitive materials benefit from being protected from atmospheric oxygen. A wide variety of multilayer laminate structures has been developed to provide barrier properties and other performance characteristics suited to a pack's purpose. These laminates may be any combination of plastic, metal or cellulosic substrates, and may include one or more coating or adhesive layers. Laminates which include polymeric films having metals or inorganic compounds, such as silicon oxides, deposited thereon have been found to give good general barrier properties and are widely used. For many purposes, it is desirable that the coating should have a covering as well as a substrate. Laminate materials in which the gas barrier coating is to be sandwiched between two films of a laminate in this way, are referred to as adhesively formed laminates. In addition to providing good gas barrier properties, good bond strength between the films and the coating is important in adhesively formed laminates.
PVDC-Based Barrier Coatings
Most commercially applied polyvinyladene chloride (PVDC) barrier layers are applied at relatively high film weights, with film weights of greater than 1.0 gsm being typical. For example, JP 62-047716B describes applying a PVDC coating to a treated polyester film, followed by adhesive lamination to a poly(ethylene) film. An oxygen barrier of 8.3 cm3/m2/day and a bond strength of 6.6 N/15 mm were recorded. Here, the oxygen barrier was achieved with a dry film weight of the PVDC of around 3 gsm (dry).
Vapor Deposited Inorganic Films
The use of vapor deposition techniques to apply silicon oxide, aluminum oxide and aluminum layers to film surfaces is well known and both excellent barrier and bond strengths are possible. Sol-gel type compositions comprising solutions of polyvinyl alcohol (PVOH) and/or ethylene vinyl alcohol (EVOH) and hydrolyzed alkoxy-silanes may be applied to the surface of the inorganic layer prior to lamination. These coatings not only enhance the barrier performance of the inorganic layer but also provide a degree of protection during printing and lamination, since these inorganic layers are very fragile. Due to the poor flex resistance of these inorganic layers these additional sol-gel coatings confer a degree of improved barrier after these types of laminates have been flexed and/or folded.
JP 2007223286 discloses coating an AlOx coated nylon film with a sol-gel coating of the type described above. When this was adhesively laminated to a PE film an oxygen barrier of 4.2 cm3/m2/day and a bond strength of 10.5 N/15 mm was achieved. JP 2005256061 discloses the printing and adhesive lamination of PET-AlOx/SiOx films to deliver oxygen barrier of 15 cm3/m2/day and a bond strength of 1.8 N/cm.
PVOH Organic Composite Coatings
WO 2007034943 describes a coating comprising both PVOH and an ethylene-maleic anhydride copolymer being applied to a nylon film followed by heat treatment (at up to 220° C.). When adhesively laminated to a heat-sealable film the laminate provided a barrier of 18.6 cm3/m2/day and a lamination bond strength of 4.0 N/cm. The high temperature treatments required in order to deliver performance are not accessible to the majority of printers and converters and thus the usefulness of for this kind of coating is limited.
Oxygen Barrier Coatings Based on PVOH/EVOH and Clay
Gas barrier coatings comprising dispersed clay, especially nanoparticles, and a hydrophilic polymer, such as polyvinyl alcohol (PVA) or ethylene-vinyl alcohol copolymer (EVOH), have been used previously. However, it has proved difficult to formulate and apply such coatings in an efficient manner that results in both good bond strengths between the flexible plastics films and adequate gas barrier properties. Examples of these compositions are disclosed in EP 0590263 B1; U.S. Pat. No. 4,818,782; EP 0479031 A; and JP-A-1313536. Typically an anchor or primer layer is required for this type of barrier coating to deliver good bond strengths in adhesively formed laminates. For example, JP-A-2007136984 (Toppan) discloses the application of EVOH-Clay composite coatings to a base layer having an anchor coat, prior to adhesive lamination to a second plastic film. The examples describe both polypropylene and polyester based webs, both requiring the use of an anchor coat prior to application of the EVOH-Clay composite. JP-A-2007136984 describes a decrease in adhesive lamination bond strength as the clay content increases.
WO 2009098463 A1 (SunChemical) describes how PVOH/EVOH-clay composite coatings are advantageously delivered as 2-pack compositions to ensure that satisfactory bond strengths were achieved in adhesively formed laminates.
Oxygen Barrier Coatings Containing Polycarboxylic Acid Polymers
JP 11-246729 (Sumitomo) discloses a resin composition containing polyvinyl alcohol, a water-soluble polyacrylic acid system compound, and an inorganic laminar compound. The resin composition is obtained by processing polyvinyl alcohol, a water-soluble polyacrylic acid system compound, and an inorganic laminar compound with a high pressure dispersion apparatus. JP 11-246729 discloses that the inorganic laminar compound may be mixed with water prior to combining with a solution of polymers. However, in order to fully disperse the inorganic laminar compound in the resin composition, processing of such a mixture through high pressure dispersion apparatus is required. The application of the coating to a polyester film and subsequently laminating it to a LLDPE film is described.
U.S. Pat. No. 6,709,735 B2/EP 1 451 008 B1 and U.S. Pat. No. 6,991,837 B2 (Mitsubishi) disclose the use of compositions of PVOH and copolymers of acrylic acid and maleic acid with a molecular weight of from about 3500 to about 5000 to prepare barrier coatings.
Barrier Coatings with Good Barrier Properties at High RH
U.S. Pat. No. 7,521,103 B2 (Mitsubishi) discloses compositions including copolymers of vinyl alcohol and vinyl amine. The use of this copolymer is shown to provide a superior barrier to that in the earlier patents particularly at raised RH.